1. Field of the Invention
The present invention relates to a projection optical system, an exposure apparatus, and a device manufacturing method.
2. Description of the Related Art
To improve resolution, a double patterning technique is proposed. The double patterning technique is a technique for dividing a fine pattern which cannot be resolved by one time exposure so that the fine pattern can be resolved, and separately exposing the divided patterns.
Since a plurality of exposures are required to obtain a desired pattern, a use of the double patterning technique causes a decrease in device productivity. Therefore, productivity improvement is required for an exposure apparatus using the double patterning technique. To improve productivity of the exposure apparatus, the power of an exposure light source may be increased. However, when the power of the exposure light source is increased, an exposure load imposed on the exposure apparatus also increases. Especially, when an exposure load is applied to a projection optical system, there occurs a refractive-index change of an optical element included in the projection optical system, or a surface shape deformation of the optical element, by an exposure heat. As a result, an aberration of the projection optical system changes, and an imaging performance of the projection optical system deteriorates.
Therefore, it is proposed to correct the aberration (exposure aberration) of the projection optical system caused by the exposure load by driving the optical element included in the projection optical system in an optical axis direction, or tilting the optical element relative to the optical axis. In Japanese Patent Application National Publication (Laid-Open) No. 2002-519843, a method is discussed in which the exposure aberration is corrected by deforming the optical element included in the projection optical system. The exposure aberration is corrected by arranging a plurality of piezoelectric elements around a lens and deforming the lens by the piezoelectric elements. In Japanese Patent Application Laid-Open No. 10-039208, a method is discussed in which an aberration due to manufacturing errors is corrected by deforming a lens near an aperture stop of the projection optical system or a lens near a surface of an object. In Japanese Patent Application National Publication (Laid-Open) No. 2008-546007, a method is discussed in which a higher-order aberration is corrected by deforming a lens in the projection optical system.
In recent years, a higher-order asymmetrical aberration in the pupil of the projection optical system (higher-order in-pupil asymmetrical aberration) that depends on a position in a field of view (that depends on an image height) is not ignorable. When the higher-order in-pupil asymmetrical aberration is large, overlay accuracy deteriorates. The higher-order in-pupil asymmetrical aberration is an aberration of (2m−1)-fold rotational symmetry (m≧4) in the pupil of the projection optical system.
By driving the lens in the optical axis direction or tilting the lens as in conventional techniques, only a low-order aberration can be corrected. By deforming the lens near the aperture stop as in Japanese Patent Application National Publication (Laid-Open) No. 2002-519843 and Japanese Patent Application Laid-Open No. 10-039208, the higher-order aberration can be corrected. However, the correction is limited to an aberration component independent of the image height. Further, only an aberration component corresponding to the deformation component applied to the lens can be corrected. By deforming the lens near the surface of the object as in Japanese Patent Application Laid-Open No. 10-039208, only a distortion or a field curvature can be mainly corrected. Although, in Japanese Patent Application National Publication (Laid-Open) No. 2008-546007, a higher-order aberration is corrected, a plurality of lenses are deformed to control rates of a plurality of aberrations to be corrected. When deforming a plurality of lenses, a severe accuracy of deformation is required and a lens barrel structure of the projection optical system becomes complex.